.TH ARP 8 "2008\-10\-03" "net\-tools" "Linux System Administrator's Manual"
.SH NAME
arp \- manipulate the system ARP cache
.SH SYNOPSIS
.B arp
.RB [ \-vn ]
.RB [ \-H
.IR type ]
.RB [ \-i
.IR if ]
.RB [ \-ae ]
.RI [ hostname ]
.PP
.B arp
.RB [ \-v ]
.RB [ \-i
.IR if ]
.B \-d
.I hostname
.RB [ pub ]
.PP
.B arp
.RB [ \-v ]
.RB [ \-H
.IR type ]
.RB [ \-i
.IR if ]
.B \-s
.I hostname hw_addr
.RB [ temp ]
.PP
.B arp
.RB [ \-v ]
.RB [ \-H
.IR type ]
.RB [ \-i
.IR if ]
.B \-s
.I hostname hw_addr
.RB [ netmask
.IR nm ]
.B pub
.PP
.B arp
.RB [ \-v ]
.RB [ \-H
.IR type ]
.RB [ \-i
.IR if ]
.B \-Ds
.I hostname
.I ifname
.RB [ netmask
.IR nm ]
.B pub
.PP
.B arp
.RB [ \-vnD ]
.RB [ \-H
.IR type ]
.RB [ \-i
.IR if ]
.B \-f
.RI [ filename ]

.SH DESCRIPTION
.B Arp
manipulates or displays the kernel's IPv4 network neighbour cache. It can add
entries to the table, delete one or display the current content.

.B ARP
stands for Address Resolution Protocol, which is used to find the media
access control address of a network neighbour for a given IPv4 Address.
.SH MODES
.B arp
with no mode specifier will print the current content of the table. It is
possible to limit the number of entries printed, by specifying an hardware
address type, interface name or host address.

.B arp -d
.I address
will delete a ARP table entry. Root or netadmin privilege is required to do
this. The entry is found by IP address. If a hostname is given, it will be
resolved before looking up the entry in the ARP table.

.B arp -s
.I address hw_addr
is used to set up a new table entry. The format of the
.I hw_addr
parameter is dependent on the hardware class, but for most classes one can
assume that the usual presentation can be used.  For the Ethernet class,
this is 6 bytes in hexadecimal, separated by colons. When adding proxy arp
entries (that is those with the
.BR pub lish
flag set) a
.B netmask
may be specified to proxy arp for entire subnets. This is not good
practice, but is supported by older kernels because it can be
useful. If the
.B temp
flag is not supplied entries will be permanent stored into the ARP
cache. To simplify setting up entries for one of your own network interfaces, you can use the
.B "arp \-Ds"
.I address ifname
form. In that case the hardware address is taken from the interface with the
specified name.

.br
.SH OPTIONS
.TP
.B "\-v, \-\-verbose"
Tell the user what is going on by being verbose.
.TP
.B "\-n, \-\-numeric"
shows numerical addresses instead of trying to determine symbolic host, port
or user names.
.TP
\fB\-H\fI type\fR, \fB\-\-hw\-type\fI type\fR, \fB\-t\fI type\fR
When setting or reading the ARP cache, this optional parameter tells
.B arp
which class of entries it should check for.  The default value of
this parameter is
.B ether
(i.e. hardware code 0x01 for IEEE 802.3 10Mbps Ethernet).
Other values might include network technologies such as
.RB "ARCnet (" arcnet ")"
,
.RB "PROnet (" pronet ")"
,
.RB "AX.25 (" ax25 ")"
and
.RB "NET/ROM (" netrom ")."
.TP
.B \-a
Use alternate BSD style output format (with no fixed columns).
.TP
.B \-e
Use default Linux style output format (with fixed columns).
.TP
.B "\-D, \-\-use-device"
Instead of a hw_addr, the given argument is the name of an interface.
.B arp
will use the MAC address of that interface for the table entry. This is usually the best option to set up a proxy ARP entry to yourself.
.TP
\fB\-i\fI If\fR, \fB\-\-device\fI If\fR
Select an interface. When dumping the ARP cache only entries matching
the specified interface will be printed. When setting a permanent or
.B temp
ARP entry this interface will be associated with the entry; if this
option is not used, the kernel will guess based on the routing
table. For
.B pub
entries the specified interface is the interface on which ARP requests will
be answered.
.br
.B NOTE:
This has to be different from the interface to which the IP
datagrams will be routed.
.B NOTE:
As of kernel 2.2.0 it is no longer possible to set an ARP entry for an
entire subnet. Linux instead does automagic proxy arp when a route
exists and it is forwarding. See
.BR arp (7)
for details. Also the
.B dontpub
option which is available for delete and set operations cannot be
used with 2.4 and newer kernels.
.TP
\fB\-f\fI filename\fR, \fB\-\-file\fI filename\fR
Similar to the
.B \-s
option, only this time the address info is taken from file
.IR filename .
This can be used if ARP entries for a lot of hosts have to be
set up.  The name of the data file is very often
.IR /etc/ethers ,
but this is not official. If no filename is specified
.I /etc/ethers
is used as default.
.sp 1
The format of the file is simple; it
only contains ASCII text lines with a hostname, and a hardware
address separated by whitespace. Additionally the
.BR "pub" , " temp" " and" " netmask"
flags can be used.
.LP
In all places where a
.B hostname
is expected, one can also enter an
.B "IP address"
in dotted-decimal notation.
.P
As a special case for compatibility the order of the hostname and
the hardware address can be exchanged.
.LP
Each complete entry in the ARP cache will be marked with the
.B C
flag. Permanent entries are marked with
.B M
and published entries have the
.B P
flag.
.SH EXAMPLES
.B /usr/sbin/arp -i eth0 -Ds 10.0.0.2 eth1 pub

This will answer ARP requests for 10.0.0.2 on eth0 with the MAC address for
eth1.

.B /usr/sbin/arp -i eth1 -d 10.0.0.1

Delete the ARP table entry for 10.0.0.1 on interface eth1. This will match
published proxy ARP entries and permanent entries.
.SH FILES
.I /proc/net/arp
.br
.I /etc/networks
.br
.I /etc/hosts
.br
.I /etc/ethers
.SH SEE ALSO
.BR ethers (5),
.BR rarp (8),
.BR route (8),
.BR ifconfig (8),
.BR netstat (8)
.SH AUTHORS
Fred N. van Kempen <waltje@uwalt.nl.mugnet.org>, Bernd Eckenfels <net\-tools@lina.inka.de>.
